IIT JEE Electrostatics Study Material

Electrostatics is a vital branch of Physics. It is an interesting branch and questions are often asked from it in the JEE. It is important to have a strong grip on the topics of electrostatics in order to remain competitive in the JEE.

Introduction:-

The Greek word for amber is “elektron”; this is the origin of the terms electricity and electron. Electrostatic is a branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges with no acceleration.

While it’s hard to see the electric charges that are responsible for electricity, it’s easy to see their effects. They’re all around us: in the sparks and shocks of a cold winter day, the imaging process of a xerographic copier, and the illumination of a flashlight when you turn on its switch. Although we often take electricity for granted, it clearly underlies many aspects of our modern world.

Just imagine what life would be like if there were no electric charges and no electricity. For starters, we’d probably be sitting around campfires at night, trying to think of things to do without television, cell phones, or computer games. But before you remark on just how peaceful such a pre-electronic-age existence would be, let me add one more sobering thought: we wouldn’t exist either. Whether it’s motionless as static charge or moving as electric current, electricity really does make the world go ‘round.

Electricity may be difficult to see, but you can easily observe its effects. How often have you found socks clinging to a shirt as you remove them from a hot dryer or struggled to throw away a piece of plastic packaging that just won’t leave your hand or stay in the trash can? The forces behind these familiar effects are electric in nature and stem from what we commonly call “static electricity.” Static electricity does more than just push things around, however, as you’ve probably noticed while reaching for a doorknob or a friend’s hand on a cold, dry day. In this section, we’ll examine static electricity and the physics behind its intriguing forces and often painful shocks.

When a plastic comb is rubbed with your hairs, it acquires the property of attracting light objects such as paper pieces.

EXPERIMENT (Moving Water without Touching It):-

Unlike gravity, which always pulls objects toward one another, electric forces can be either attractive or repulsive. You can experiment with electric forces using a thin stream of water and an electrically charged comb. First, open a water faucet slightly so that the flow of water forms a thin but continuous strand below the mouth of the faucet. Next, give your rubber or plastic comb an electric charge by passing it rapidly through your hair or rubbing it vigorously against a wool sweater. Finally, hold the comb near the stream of water, just below the faucet, and watch what happens to the stream. Is the electric force that you’re observing attractive or repulsive? Why does this force change the path of the falling water? Rubbing the comb through your hair makes it electrically charged. What other objects can acquire and hold a charge when you rub them across hair or fabric? Which works better: a metal object or one that’s an insulator? Why?

Charge:-

“Charge” is the technical term used to indicate that an object has been prepared so as to participate in electrical forces.” This is to be distinguished from the common usage, in which the term is used indiscriminately for anything electrical. For example, although we speak colloquially of “charging” a battery, you may easily verify that a battery has no charge in the technical sense, e.g., it does not exert any electrical force on a piece of tape that has been prepared.

There are two types of electric charge, called positive and negative. The subatomic particle called a proton has a positive charge, and an electron has a negative charge.

Charge comes in quantized units. All protons carry the same amount of charge +e, and all electrons carry a charge -e. We will discuss how charge is measured and the unit of electric charge below.

Like charges repel each other, unlike charges attract. The electric force between two objects is repulsive if the objects carry “like” charge, that is, if both are positively charged or both are negatively charged. The electric force is attractive if the two objects carry “unlike” charge. Here the terms like and unlike refer to the signs of the charges, not their magnitudes. So, the expression “like charges” means that the two charges are both positive or both negative.

The expression “unlike charges” means that one charge is positive and the other is negative.

Charge is conserved. The total charge on an object is the sum of all the individual charges (protons and electrons) carried by the object. The total charge can be positive, negative, or zero. Charge can move from place to place, and from one object to another, but the total charge of the universe does not change.

View this video for more on electrostatics:-

INSULATORS, CONDUCTORS AND SEMICONDUCTORS:-

Substances can be classified in terms of their ability to conduct electric charge.

Conductor:-

In conductors, electric charges move freely in response to an electric force. All other materials are called insulators and semiconductors.

Insulator:-

Glass and rubber are insulators. When such materials are charged by rubbing, only the rubbed area becomes charged, and there is no tendency for the charge to move into other regions of the material. In contrast, materials such as copper, aluminium, and silver are good conductors. When such materials are charged in some small region, the charge readily distributes itself over the entire surface of the material.

If you hold a copper rod in your hand and rub the rod with wool or fur, it will not attract a piece of paper. This might suggest that a metal can’t be charged. However, if you hold the copper rod with an insulator and then rub it with wool or fur, the rod remains charged and attracts the paper. In the first case, the electric charges produced by rubbing readily move from the copper through your body and finally to ground. In the second case, the insulating handle prevents the flow of charge to ground.

Semiconductor:-

Semiconductors are a third class of materials, and their electrical properties are somewhere between those of insulators and those of conductors. Silicon and germanium are well-known semiconductors that are widely used in the fabrication of a variety of electronic devices.

When a rod of plastic is rubbed with fur or a glass rod is rubbed against silk, then it is generally observed that the rods start attracting some pieces of paper and seem to be electrically charged. While the charge on plastic is defined to be negative, that on silk is considered positive. The vast amount of charge in an everyday object is usually hidden, comprising equal amount of two kinds – positive and negative. The imbalance is always small compared to the total amounts of positive charge and negative charge contained in the object.

Some general conceptual question:-

Question 1: The gift you are about to unwrap is electrically neutral. You tear off the clingy wrapper and find that it has a large negative charge. What charge does the gift itself have, if any?

Answer: It has a large positive charge equal in amount to the wrapper’s negative charge.

Why: Since charge is a conserved physical quantity, the wrapper and gift must remain neutral overall even after you separate them. The wrapper’s negative charge must be balanced by the gift’s positive charge.

Question 2 : When you peel a piece of adhesive tape off a glass window, you find that the tape is attracted toward the spot it left behind. How did the tape and glass acquire electric charges?

Answer: While the tape and glass were in contact, charge was unevenly distributed between their surfaces. Removing the tape merely made that imbalance more obvious.

Why: The tape and glass have different chemical affinities for electrons and become oppositely charged whenever they touch. In fact, the tape’s stickiness itself comes from electrostatic attraction.

Question 3 : Although any cloud may contain opposite charges, only the violent updrafts inside thunderheads are able to separate those charges and produce lightning. Why does such separation lead to lightning?

Why: When opposite charges are nearby, they don’t necessarily have much electrostatic potential energy per charge and the voltages may be small. Separating those charges to great distances dramatically increases their stored energy and produces high voltages.

Question 4 : The paper in some printing presses moves through the rollers at half a kilometre per minute. If no care is taken, dangerous amounts of static charge can accumulate on parts of the press. How does the moving paper contribute to that charging process?

Answer: Contact between dissimilar materials puts charge on the paper, which then carries that charge with it to isolated parts of the press. Enough charge can accumulate on those parts to be dangerous.

Why: Nonconductive paper is an excellent transporter of electric charge. Once the paper picks up a static charge by touching a dissimilar material, it can carry that charge with it as it moves through the press. Not surprisingly, printing presses use various tools to suppress this static charging.

Question 5 : The conveyor belts used to move flammable materials often have metal threads woven into their fabric. Why are such conducting belts important for fire safety?

Answer: An insulating conveyor belt can separate enormous amounts of charge, leading to high voltages, sparks, and possibly fire. A conductive belt can’t carry charge with it as it moves, so no charge accumulates.

Why: When an insulating belt has charge on its surface, that charge must move with the belt. However, charges are mobile in a conductive belt and don’t normally move with it.

Only the negatives (elelctrons) move in solid conductors.

In a neutral object there are equal numbers of positives and negatives.

A neutral object has no net charge. (Note: charge is a conserved quantity, so we can't create new charge in an interaction, just move some around.)

The lines of force are directed away from a positively charged conductor and are directed towards a negatively charged conductor.

A line of force starts from a positive charge and ends on a negative charge. This signifies line of force starts from higher potential and ends on lower potential.

Question 1:-

Which are the charged parts of an atom.

(a) Only electrons

(b) Only protons

(c) Neutrons only

(d) Electrons and neutrons

(e) Electrons and protons

(f) Protons and neutrons

Question 2:-

Electrical forces

(a) can cause objects to only attract each other

(b) can cause objects to only repel each other

(c) can cause objects to attract or repel each other

(d) have no effect on objects

Question 3:-

A rubber balloon possesses a positive charge. If brought near and touched to the door of a wooden cabinet, it sticks to the door. This does not occur with an uncharged balloon. These two observations can lead one to conclude that the wall is _____

(a) electrically neutral

(b) negatively charged

(c) a conductor

(d) lacking electrons

Question 4:-

In case the charges are similar the force is of?

(a) repulsion

(b) zero value

(c) maximum value

(d) attraction

Question 5:-

Which of the following materials are likely to exhibit more conductive properties than insulating properties? _____ Explain your answers.